💧 French Drain Calculator
Calculate materials needed for French drain installation
Total length of French drain trench
How to Use This Calculator
Enter Trench Length
Input the total length of your French drain trench in feet. Measure the path where you'll install the French drain from start to finish.
Specify Trench Dimensions
Select trench width (standard is 12 inches) and depth (standard is 18 inches, or below frost line). Deeper trenches provide better drainage but require more materials.
Select Pipe Diameter
Choose your drain pipe diameter: 4 inches (standard for most applications) or 6 inches (for larger drainage needs). Use perforated drain pipe.
Calculate and Order
Click "Calculate French Drain" to see gravel needed (in cubic yards), pipe length, and filter fabric. Add 10% extra for waste. Order perforated drain pipe and crushed stone gravel.
Formula
Trench Volume = Length × Width × Depth (all in feet)
Pipe Volume = π × (Pipe Radius)² × Length
Gravel Volume = Trench Volume - Pipe Volume
Cubic Yards = Gravel Volume (cubic feet) ÷ 27
Gravel Weight = Cubic Yards × Density (1.3 tons/yd³)
Example 1: Standard French Drain (50 ft length, 12" width, 18" depth, 4" pipe)
Step 1: Convert dimensions: 12" = 1 ft, 18" = 1.5 ft, 4" pipe = 0.33 ft diameter
Step 2: Calculate trench volume: 50 × 1 × 1.5 = 75 cubic feet
Step 3: Calculate pipe volume: π × (0.33/2)² × 50 = π × 0.027 × 50 = 4.24 cubic feet
Step 4: Calculate gravel: 75 - 4.24 = 70.76 cubic feet
Step 5: Convert to yards: 70.76 ÷ 27 = 2.62 cubic yards
Step 6: Add waste: 2.62 × 1.10 = 2.88 cubic yards (with 10% waste)
Example 2: Large French Drain (100 ft length, 16" width, 24" depth, 6" pipe)
Step 1: Convert dimensions: 16" = 1.33 ft, 24" = 2 ft, 6" pipe = 0.5 ft diameter
Step 2: Calculate trench volume: 100 × 1.33 × 2 = 266 cubic feet
Step 3: Calculate pipe volume: π × (0.5/2)² × 100 = π × 0.0625 × 100 = 19.63 cubic feet
Step 4: Calculate gravel: 266 - 19.63 = 246.37 cubic feet
Step 5: Convert to yards: 246.37 ÷ 27 = 9.13 cubic yards
About French Drain Calculator
The French Drain Calculator is an essential tool for homeowners, landscapers, contractors, and DIY enthusiasts who need to accurately determine how much gravel, pipe, and filter fabric is required for their French drain installation projects. This calculator eliminates guesswork by providing precise material calculations based on trench dimensions and pipe size, helping you order the right amount of materials without waste or shortages.
When to Use This Calculator
- Basement Waterproofing: Calculate materials needed for French drains to prevent basement flooding
- Yard Drainage: Determine materials for French drains to manage yard water and prevent standing water
- Foundation Drainage: Estimate materials for foundation French drains to protect foundation walls
- Gutter Runoff: Calculate materials for French drains to redirect gutter water away from foundation
- Budget Planning: Calculate material costs by determining exact quantities needed
Why Use Our Calculator?
- ✅ Accurate Measurements: Precise calculations prevent over-ordering or running short
- ✅ Gravel Volume: Calculates gravel needed accounting for pipe volume
- ✅ Multiple Units: Shows results in cubic yards, cubic feet, and tons
- ✅ Filter Fabric: Estimates filter fabric needed for wrapping pipe and trench
- ✅ Cost Planning: Helps budget accurately by determining exact material needs
- ✅ Time Savings: Instant calculations eliminate manual math and measurements
Common Applications
Basement Waterproofing: Homeowners planning French drains around basements use this calculator to order the correct quantity of gravel, perforated drain pipe, and filter fabric. Standard French drains use 4-inch perforated pipe in 12-inch wide trenches at 18-24 inches deep.
Yard Drainage: Landscapers and homeowners planning yard drainage systems use this tool to estimate materials needed. French drains effectively redirect water away from problem areas and prevent standing water.
Foundation Drainage: Contractors planning foundation drainage systems use this calculator to determine material quantities needed, ensuring sufficient materials are on-site before starting work.
Tips for Best Results
- Trench Depth: Standard depth is 18-24 inches; below frost line in cold climates (30-48 inches)
- Trench Width: Standard is 12 inches; wider (16 inches) for larger drainage needs
- Pipe Type: Use perforated drain pipe (allows water to enter); solid pipe for outlet only
- Gravel Type: Use crushed stone gravel (not rounded pea gravel) for better drainage
- Filter Fabric: Wrap pipe and trench with filter fabric to prevent clogging
- Add Waste: Always add 10% extra for waste, compaction, and mistakes during installation
Frequently Asked Questions
How much gravel do I need for a 50-foot French drain?
For a 50-foot French drain with 12-inch wide, 18-inch deep trench and 4-inch pipe, you'll need approximately 2.62 cubic yards of gravel. Add 10% extra for waste, so plan for about 2.88 cubic yards or 3.7 tons. Also order 50 feet of perforated drain pipe and about 120 square feet of filter fabric.
What type of pipe should I use for a French drain?
Use perforated drain pipe (with holes) for the main French drain to allow water to enter. Use solid pipe only for the outlet where water exits. Perforated pipe is typically 4-6 inches in diameter. PVC or corrugated plastic pipe both work well.
How deep should a French drain be?
Standard French drain depth is 18-24 inches for most applications. In cold climates, dig below the frost line (typically 30-48 inches deep) to prevent freezing. Deeper drains provide better drainage but require more materials.
Do I need filter fabric for a French drain?
Yes! Filter fabric (geotextile) is essential for preventing soil from clogging the gravel and pipe. Wrap the pipe and trench with filter fabric to keep dirt out while allowing water to flow through. Without filter fabric, French drains clog quickly.
What type of gravel should I use?
Use crushed stone gravel (angular, not rounded) for French drains. Crushed stone provides better drainage than rounded pea gravel. Common sizes are 3/4 inch or 1-2 inch crushed stone. Avoid using sand or fine materials that can clog the drain.
How much does a French drain cost?
Material costs vary: crushed stone $30-$50 per cubic yard, perforated pipe $1-$3 per linear foot, filter fabric $0.50-$1 per square foot. For a 50-foot French drain, materials cost approximately $150-$300. Labor costs are additional if hiring a contractor.